The present invention relates to radio telephone systems such as cordless telephones which perform communication with the remote caller (opposite side) through connection a public telephone line or network by radio communication between a slave telephone set and its master telephone set.
In the radio communication system, communication with its master telephone set is performed through the connection of the slave telephone set to the public telephone network for communication with the opposite side by the radio communication. Radio communication systems include analog ones such as cordless telephone sets utilizing analog radio communication and digital ones such as PHSs (personal handy phone systems).
In the PHS, it is possible to use the same slave telephone set (terminal) for communication both indoors (i.e., in home or in office) and outdoors. Indoors, the slave telephone set can be connected to the public telephone network by its master telephone set. Outdoors, it can be connected to the public telephone network via a base station installed by a dealer. In the PHS, it is also possible that slave telephone sets be used for communication between them just like transceivers without agency of any base station.
It is prescribed as standard to register ID codes for identifying units in both the master and slave telephone sets in the radio communication system. In the manufacture of the master and slave telephone sets, unit IDs are each registered in each of the master and slave units. Unit IDs preliminarily registered in slave units are registered in master units by registering operation, and unit IDs preliminarily registered in the master telephone sets are registered in the slave units. Unless unit IDs are identical, it is impossible to start radio communication.
The mutual recognition of the units of the master and slave telephone sets with unit IDS, is performed in order to prevent erroneous line connection between master and slave telephone sets of different users, such as an erroneous call to the own telephone set instead of a set in the next-door house or to the next-door house set instead of the own set.
In the radio communication system, a plurality of slave telephone sets can make radio communication to a single master telephone set. The master telephone set with registered unit IDs of a plurality of slave telephone sets, can discriminate each slave telephone set with the unit ID thereof, thus controlling the radio communication from each slave unit.
An operational example of radio communication using a channel between a master telephone set and a slave telephone set in a prior art radio telephone system will now be described. FIG. 3 is a view showing a control sequence in the prior art radio telephone system. The master and slave telephone sets in the radio telephone system are waiting with an out-of-use control channel found at hand for use at any time.
When the master receives the line connection request and the unit ID, it checks whether the received unit ID is of its own slave telephone set. If the two unit IDs are identical, the master telephone set finds a vacant communication channel, and assigns the found vacant communication channel n to the slave telephone set.
When the communication channel n is assigned, the slave telephone set checks that the channel n is vacant. If it is O.K., the slave telephone set transmits a confirmation signal. When the master telephone set receives the confirmation signal from the slave telephone set, it executes an operation of line connection to the public telephone network (i.e., a switch). When the line connection is completed, the master telephone set connects the line to the slave telephone set via the communication channel n.
Subsequently, like the operation in the ordinary telephone set, the telephone number of the opposite side is dialed, and the communication is started. When on-hook is performed on the slave telephone set side, a communication “off” request is transmitted from the slave telephone set to the master telephone set. In response to receipt of the communication “off” request, the master telephone set transmits a communication “off” signal to the slave telephone set, thus ending the communication. Then, both the master and slave telephone sets restore their stand-by state again.
As described above, in the prior art radio telephone system the master telephone set assigns a communication channel to the slave telephone set for the radio communication by recognizing the slave telephone set based on a unit ID, which is one-to-one correspondence to one slave telephone set. That is, the radio communication of the slave telephone set is controlled with the unit ID in one-to-one correspondence to one slave telephone set. In other words, it is contemplated only to assign a single communication channel to one slave telephone set for the radio communication, and it is not considered to assign two or more channels for the radio communication.
If it is intended to assign two or more communication channels for communication to one standard slave telephone set, for which only a single unit ID is preliminarily registered as noted above, the master telephone set has to control radio communication with its own control system in addition to the radio communication control with the unit ID since the master telephone set can not control a plurality of communication channels by one unit ID. This posed a problem that the unit construction of the radio communication system (or master telephone set) is complicated and the slave unit control operation becomes more cumbersome.
Moreover, in the prior art radio communication system, in case when a plurality of slave stations simultaneously start communication with one master telephone set by using two or more communication channels, the traffic (i.e., line utilization status) is extremely increased because usually at most three or four channels are provided as radio lines. This leads to a problem of readier generation of a status, in which radio communication between the master telephone set and a slave telephone set is difficult.